What Causes Bleeding, Stomach Cramps, Clot Like Discharge After Taking Pills?

I am on the pill and have taken a few packs back to back but 2 days ago I got some bleeding and have had mild stomach cramps, it seemed a bit like my period had come at an unexpected time but today I had a bit of strange clot-like discharge not like my usual period. As far as Im aware I wasn't pregnant, but is this a miscarriage and what should I do?